Donzi, Thank You. Very helpful. A couple more questions, How hot does it get? I am mounting mine on the transom and have a little concern about the heat. The 3/8" npt plug? What is that for?

I took the Hardin Crossover Thermostat set up off and went back to the factory style t stat and recirculation pump, so now oil temps come up much faster and consistent. With the crossover the motor would hardly make coolant temp and my oil temp would hardly hit 180 after being ran hard. Now with a 140 t stat in the cooling set up and recirculation pump it will hit 190 deg at cruising speeds (warm freshwater lakes/ Texas summer) and will hit 210 when ran hard for an extended time. This seems to be inline with most targeted oil temps everyone on here shoots for. The plug on top is 1/2 NPT and I verified that on the mercury parts schematics, so it must have been there from the factory and I'm not sure what it would be used for. As far as the housing it gets hot enough you can't put your hand on it (provably why it has the idiot sticker). I have shot my filter with an infrared temp gun (cheapie probably +/- 10 degrees or more at distance from the helm) and its reads in the 170's if I recall, I assume the housing gets similar temps just never checked. I'm assuming merc put it on the back of block/ head for that reason.

Thanks again for the info.
I wish I knew what that 1/2" plug is for. Can I put a gauge there, is it oil from engine or from cooler? Can anybody help with this mystery?

Brain Trust Help Needed.
How do you test the thermostat in this unit? Mine seems to go from the motor through the cooler back to the filter then to the block.
This is blowing thru the fittings, seems at room temperature it should go thru the filter and bypass the cooler. Am I missing something here or does it take oil pressure to activate the thermostat assembly? Also shouldn't it go thru the filter then the cooler?
ANY help appreciated.

As you say lots of crickets, but here is how it should work

Oil pump to outside of filter (supply) then inside of filter to block/bearings. The Oil like all things flows the path of least resistance, so when temps are cool the oil will flow thru everything, but mostly (not all) will bypass the cooler. When oil reaches temp of the thermostat, the stat retracts and closes off or at least makes it harder to bypass cooler.

To verify stat is working is by measuring temps at the lines as it opens and closes

The plug on top should be used to bleed air out and or install a gauge.

From improved drag racing.com

https://cimg1.ibsrv.net/gimg/www.off...aae1a1ca83.png

When I blow through mine (Oil into filter fitting) NO air goes to filter ALL air goes to Cooler. That is what I do not understand.
Does it need oil pressure to actuate?
I see a circle clip holds it together.

Lines not routed correctly maybe, or thermostat stuck

Lines are correct. Does this thermostat fault to open? How do you remove it?

Don't really know how the Merc stat is made, but could be its only blocking one side (supply) which goes into the cooler. If lines are feeding the return side of the stat then it will supply oil/air to cooler lines and filter will be an issue. Just something of what I'm think about


https://cimg5.ibsrv.net/gimg/www.off...5a9bca894e.jpg

Don't know the default, but need to remove a snap ring (item 6) to get at thermostat

https://www.mercruiserparts.com/bam/...31331/5389/170